1. Field of the Invention
This invention relates to a dielectric ceramic composition, a multi-layer ceramic capacitor with internal copper electrodes and a method of manufacturing a multi-layer ceramic capacitor with internal copper electrodes.
2. Description of the Prior Art
Recently, multi-layer ceramic capacitors have been widely used in the field of microelectronics and hybrid circuitry due to its compactness, large capacitance and high reliability.
In the manufacture of multi-layer ceramic capacitors, the dielectric ceramic body and internal electrodes are cofired. Conventional multi-layer ceramic capacitors have used barium titanate (BaTiO.sub.3) based materials as the dielectric. Since these materials should be fired at high temperature, e.g. 1300.degree. C. in an oxidizing atmosphere, the internal electrodes are necessary to be made of noble metals such as platinum (Pt) and palladium (Pd) which are expensive. The high cost of the internal electrode materials has resulted in substantial increase in the manufacturing cost of multi-layer ceramic capacitors.
Therefore, there have been two demands for the dielectric materials for multi-layer ceramic capacitors. One demand is that the dielectric material can be sintered at a low temperature below 1100.degree. C. for enabling the use of less expensive metals such as silver-palladium alloys for internal electrodes. The other demand is that the dielectric material can be sintered in a reducing atmosphere and shows high resistivity for enabling the use of base metals such as nickel or copper.
U.S. Pat. No. 4,115,493 discloses a non-reducing BaTiO.sub.3 based dielectric ceramic composition and a multi-layer ceramic capacitor with internal nickel electrodes using the composition. The dielectric composition [(Ba.sub.1-x Ca.sub.x)O.sub.m (Ti.sub.1-y Z.sub.y)O.sub.z ] can be sintered at a temperature range of 1300.degree. to 1400.degree. C. in an atmosphere of low oxygen content and shows high electrical resistivity.
U.S. Pat. No. 4,450,240 discloses a low-firing dielectric ceramic composition of PbTiO.sub.3 -Pb(Ni.sub.1/3 Nb.sub.2/3) O.sub.3 -Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 and a multi-layer ceramic capacitor employing the composition. Since the composition can be sintered at a temperature range of 900.degree. to 1050.degree. C. in an oxidizing atmosphere, silver-palladium alloys can be used as the internal electrodes of the multi-layer ceramic capacitor. Copper can not be used for the internal electrodes of the multi-layer ceramic capacitor employing this ceramic composition because the composition can not be sintered in a reducing atmosphere. Copper is the ideal electrode material due to its high conductivity, high migration resistance and low cost.
U.S. Pat. No. 4,751,209 discloses a dielectric ceramic composition of PbTiO.sub.3 -Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 -Pb(Ni.sub.1/2 W.sub.1/2)O.sub.3 ternary system and a multi-layer ceramic capacitor using the composition. The composition can be sintered at a temperature below 1080.degree. C. and in an atmosphere of low oxygen content, which enables to manufacture a multi-layer ceramic capacitor with internal copper electrodes.
Japanese patent application laid-open No. 63-265412 discloses a multi-layer ceramic capacitor using low temperature sintering dielectric ceramic compositions and internal copper electrodes. The compositions include an oxide containing a component A and a component B and copper. The component A is selected from lead (Pb), calcium (Ca), strontium (Sr) and barium (Ba). The component B is selected from magnesium (Mg), nickel (Ni), titanium (Ti), zinc (Zn), niobium (Nb), and tungsten (W). The lead based perovskite materials have firing temperatures lower than the copper melting point, and high resistivity when fired in an oxygen deficient atmosphere.
As shown in some examples, some dielectric compositions containing lead can be sintered at a temperature below the melting point of copper in a low oxygen partial pressure atmosphere without showing a drop of the resistivity. Multi-layer ceramic capacitors made of these dielectric materials and internal copper electrodes exhibit large capacitances, high resistivities and low dielectric losses.
But there have been problems in the use of these materials. The first problem is the limitation of the sintering atmosphere. The above-mentioned dielectric materials must be sintered in an atmosphere in which oxygen partial pressure is precisely controlled at a range of 1.times.10.sup.-10 to 1.times.10.sup.-7 atm using a gas mixture, e.g. a mixture of hydrogen gas and nitrogen gas.
The second problem is the low mechanical strength of the sintered ceramic body. The above-mentioned perovskite dielectric materials containing lead have much smaller mechanical strength, e.g. 800 to 900 kg/cm.sup.2, in comparison with BaTiO.sub.3 based materials, e.g. 1300 kg/cm.sup.2. The low mechanical strength of the multi-layer ceramic capacitor body results in the problem when the chip capacitors are automatically mounted on the substrate using a chip-mounter.
The third problem is the wide grain size distribution of the sintered material. Specifically abnormal grain growth, e.g.&gt;10 .mu.m, makes it difficult to decrease the thickness of the dielectric layers in the multi-layer ceramic capacitor due to the drop of the electrical resistivity of the layers.
Meanwhile, as described earlier, in the manufacture of multi-layer ceramic capacitors, the dielectric ceramic layers and the internal electrode layers are cofired. As the organic binder contained in the dielectric green tapes prevents the ceramic body from sintering densely, the organic components should be removed completely in the binder burn-out process. When base metals are employed as internal electrodes of multi-layer ceramic capacitors, perfect removal of the organic binder is very difficult.
U.S. Pat. No. 3,902,102 discloses a fabrication process of a multi-layer ceramic capacitor whose internal electrodes are base metals. After electrode ink, containing particles of base metal, a glass frit and an organic binder ethyl cellulose, is applied on the dielectric, the green ceramic body is subsequently "post" fired in an atmosphere of low oxygen partial pressure at about 900.degree. C. After the binder burn-out step, sintering is carried out in an atmosphere, in which oxygen partial pressure is delicately controlled using CO.sub.2 -CO mixture.
U.S. Pat. No. 4,752,858 discloses a fabrication method of a multi-layer ceramic capacitor whose internal electrodes are made of copper or an alloy principally containing copper. In the method, acrylic resin is used as binder to prepare both ceramic green tapes and electrode paste. After printing electrodes, laminating the tapes and cutting, the binder burn-out process takes place in 1% O.sub.2 -N.sub.2 gas (a mixture of 1% oxygen gas and 99% nitrogen gas) at a temperature of 350.degree. C. Sintering procedure is performed using alumina tube furnace supplied with N.sub.2 -H.sub.2 gas (a mixture of nitrogen gas and hydrogen gas).
In the conventional fabrication methods of a multi-layer ceramic capacitor with base metal internal electrodes, since organic binders used for dielectric green tapes and electrode paste have properties of nondecomposition in a non-oxidizing atmosphere, it is very difficult to achieve the complete binder removal in an oxygen deficient atmosphere.